The purpose of Core A is to provide state-of-the-art measurements of reactive oxygen species (ROS) and nitric oxide (NO) in cells, tissues and membranes for participants of the PPG. CORE A will assist with preparation and processing of various biological samples for analysis using Electron Spin Resonance (ESR) spectroscopy and High Pressure Liquid Chromatography (HPLC). This Core facility has existed since 1999 and has supported studies of superoxide (O2~) and NO in vascular cells in many publications from Drs. Harrison, Griendling, Dudley and Jo (1-10). It has recently validated HPLC detection of O2* in intact tissues by monitoring formation of 2-hydroxyethidium from dihydroethidium (11). This facility will significantly enhance the work of the individual laboratories and will enhance collaborations between participants of the PPG.